This invention relates to a thin-film EL device having at least a structure comprising a lower electrode layer having a predetermined pattern, a lower insulating layer, a light emitting layer, and an upper electrode layer of a transparent conductive material stacked on an electrically insulating substrate. It also relates to a composite substrate for use in thin-film EL devices and various other display devices.
EL devices are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices utilize the phenomenon that a material emits light upon application of an electric field, known as electroluminescent phenomenon.
The EL devices using inorganic phosphors include dispersion type EL devices of the structure that a dispersion of powder phosphor in organic material or enamel is sandwiched between electrode layers, and thin-film type EL devices in which a light emitting thin film sandwiched between a pair of insulating thin films and further between a pair of electrode layers is disposed on an electrically insulating substrate. For each type, the drive modes include DC voltage drive mode and AC voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the thin-film EL devices are currently on widespread use on account of a high luminance and a long lifetime.
FIG. 2 shows the structure of a dual insulated thin-film EL device as a typical prior art EL device. This thin-film EL device has a structure comprising a lower electrode layer 3, a lower insulating layer 4, a light emitting layer 5, an upper insulating layer 6, and an upper electrode layer 7 stacked on an electrically insulating substrate 2. The substrate 2 is transparent and constructed, for example, of a soda-lime glass customarily used in liquid crystal displays and plasma display panels (PDP). The lower electrode layer 3 is a layer of indium tin oxide (ITO) having a thickness of about 0.2 to 1 xcexcm. The lower and upper insulating layers 4 and 6 are thin films deposited by sputtering, evaporation or the like to a thickness of about 0.1 to 1 xcexcm and usually formed of Y2O3, Ta2O5, Al3N4, BaTiO3 or the like. The light emitting layer 5 has a thickness of about 0.2 to 1 xcexcm. The upper electrode layer 7 is formed of a metal such as Al. The lower and upper electrode layers 3 and 7 are patterned into orthogonally extending stripes so that they constitute column and row electrodes, respectively. In this electrode matrix, the intersections between column and row electrodes make pixels. The matrix electrodes are controlled to apply an AC voltage or pulse voltage to a selected pixel whereby the light-emitting material at that site emits light which comes out from the substrate 2 side.
In this thin-film EL device, the lower and upper insulating layers 4 and 6 have a function of restricting the current flow through the light emitting layer 5 in order to restrain breakdown of the thin-film EL device and act so as to provide stable light-emitting properties. Thus thin-film EL devices of this structure find widespread commercial use.
Among phosphor materials of which the light-emitting layer 5 is made, Mn-doped ZnS exhibiting yellowish orange light emission has mainly been used for ease of film formation and light-emitting properties. For color display fabrication, it is inevitable to use light-emitting materials capable of emitting light in the three primary colors, red, green and blue. These materials known so far in the art, for instance, include Ce-doped SrS and Tm-doped ZnS exhibiting blue light emission, Sm-doped ZnS and Eu-doped CaS exhibiting red light emission, and Tb-doped ZnS and Ce-doped CaS exhibiting green light emission.
Shosaku Tanaka, xe2x80x9cthe Latest Development in Displaysxe2x80x9d in Monthly Display, April, 1998, pp. 1-10, discloses ZnS, Mn/CdSSe, etc. as red light-emitting materials, ZnS:TbOF, ZnS:Tb, etc. as green light-emitting materials, and SrS:Cr, (SrS:Ce/ZnS)n, CaGa2S4:Ce, SrGa2S4:Ce, etc. as blue light-emitting materials. Such light-emitting materials as SrS:Ce/ZnS:Mn are also disclosed as white light-emitting materials.
International Display Workshop (IDW), 1997, X. Wu, xe2x80x9cMulticolor Thin-Film Ceramic Hybrid EL Displaysxe2x80x9d, pp. 593-596 discloses that among the aforesaid materials, SrS:Ce is used as a blue light-emitting layer in a thin-film EL device. In addition, this article discloses that when a light-emitting layer of SrS:Ce is formed, an electron beam evaporation process in a H2S atmosphere enables to form a light-emitting layer of high purity.
However, for these thin-film EL devices, a structural problem remains unsolved. When a large area display is fabricated, steps appear on the lower insulating layer 4 at the edges of the pattern of the lower electrode layer 3, and dust and debris occurring during the process introduce defects into the lower insulating layer 4. Since the lower insulating layer 4 is a thin film, it is difficult to reduce to nil such steps and defects, resulting in a destruction of the light-emitting layer 5 due to a local dielectric strength drop. These problems are fatal to display devices, and become a bottleneck in the wide practical use of thin-film EL devices in a large-area display system, in contrast to liquid crystal displays or plasma displays.
To provide a solution to the defect problem associated with such thin-film insulating layers, JP-B 07-44072 discloses an EL device using an electrically insulating ceramic substrate as the substrate 2 and a thick-film dielectric layer instead of a thin-film insulating layer as the lower insulating layer 3. Since the EL device of the above patent is constructed such that light emitted by the light emitting layer 5 is extracted from the upper side remote from the substrate 2 as opposed to prior art thin-film EL devices, a transparent electrode layer is used as the upper electrode 7.
Further, in this EL device, the thick-film dielectric layer is formed to a thickness of several tens to several hundreds of microns, which is several hundred to several thousand folds of the thickness of the thin-film insulating layer. This minimizes the potential of breakdown which is otherwise caused by steps in the lower electrode layer 3 and pinholes formed by debris during the manufacturing process, ensuring advantages of high reliability and high manufacturing yields. Meanwhile, the use of such a thick-film dielectric layer entails a problem of reducing the effective voltage applied across the light emitting layer 5. For example, the above-referred JP-B 7-44072 overcomes this problem by constructing the thick-film dielectric layer from a lead-containing complex perovskite high-permittivity material.
However, the light emitting layer formed on the thick-film dielectric layer has a thickness of several hundreds of nanometers which is merely about {fraction (1/100)} of that of the thick-film dielectric layer. This requires that the thick-film dielectric layer on the surface be smooth at a level below the thickness of the light emitting layer. However, a conventional thick-film procedure is difficult to form a dielectric layer having a fully smooth surface.
Specifically, the thick-film dielectric layer is essentially constructed of a ceramic material obtained by sintering a powder raw material. Intense sintering generally brings about a volume contraction of about 30 to 40%. Unfortunately, although customary ceramics consolidate through three-dimensional volume contraction upon sintering, thick-film ceramics formed on substrates cannot contract in the in-plane directions of the substrate under restraint by the substrate, and is allowed for only one-dimensional volume contraction in the thickness direction. For this reason, sintering of the thick-film dielectric layer proceeds insufficiently, resulting in an essentially porous body. Moreover, since the surface roughness of the thick film is not reduced below the crystal grain size of the polycrystalline sintered body, its surface have asperities greater than the submicron size.
When the thick-film dielectric layer is porous or has surface asperities as mentioned above, it is impossible to deposit thereon a light-emitting layer as a uniform thin film by a vapor phase deposition technique such as evaporation or sputtering because the light-emitting layer cannot conform to the surface morphology of the thick-film dielectric layer. This raises problems such as a decrease in effective light-emitting area because an electric field cannot be effectively applied to the portions of the light-emitting layer formed on non-flat portions of the thick-film dielectric layer, and a decrease in luminance because local non-uniformity of film thickness causes a local dielectric breakdown of the light-emitting layer. Furthermore, locally large thickness fluctuations cause the strength of an electric field applied to the light-emitting layer to locally vary too largely, failing to establish a definite light emission voltage threshold.
To solve these and other problems, for example, JP-A 7-50197 discloses a procedure of improving surface smoothness by stacking on a thick-film dielectric layer of lead niobate a high-permittivity layer of lead titanate zirconate or the like which is formed by the sol-gel technique.
As mentioned above, the use of a high-permittivity thick-film dielectric layer avoids any deficiency in the thin-film insulating layer which is otherwise caused by steps at the edges of the pattern of the lower electrode layer and dust, etc. occurring in the production process, and overcomes the problems that the light-emitting layer can be destructed by a local dielectric strength drop.
For the thick-film dielectric layer, lead base dielectrics are often used in order to acquire such advantages as potential low-temperature sintering, high permittivity and high dielectric strength. On use of lead base dielectrics, however, a sintering temperature of at least 700xc2x0 C., and most often, at least 800xc2x0 C. is still needed. Moreover, since the firing of a thick-film dielectric layer is generally carried out in a high-temperature oxidizing atmosphere, the lower electrode layer formed prior to the thick-film dielectric layer should have both heat resistance and oxidation resistance. Also, when the thick-film dielectric layer is formed of a lead base dielectric material, the very high reactivity of lead oxide as one constituent of the dielectric material requires that the material of which the lower electrode layer is made have least reactivity with lead oxide at high temperature, in addition to the normal requirements of heat resistance and oxidation resistance. Since the lower electrode layer is patterned on practical use, the electrode pattern can cause steps to form on the surface of the thick-film dielectric layer if the electrode layer is very thick. This exacerbates the display quality. For this reason, it is preferred that the lower electrode layer be thin. It is thus necessary for the lower electrode layer to be formed of a material capable of providing sufficient conductivity even at a reduced thickness.
A common approach taken in the prior art to meet such property requirements is to use high-melting point noble metals as the material for the lower electrode layer. Among the noble metal electrode materials, Ag is most attractive as a high conductivity, low cost electrode material because it is very low in material cost as compared with the other noble metals including Au, Pt, Pd, Ir, Ru and Rh and has the lowest electrical resistance. However, it is difficult to use Ag alone because Ag has a low melting point and poor heat resistance as compared with the other noble metals. Then Ag is used in the form of alloys such as Agxe2x80x94Pd and Agxe2x80x94Pt as disclosed in the above-referred JP-B 7-44072 and JP-A 7-50197, and most often in the form of Agxe2x80x94Pd alloys having a Pd content of 10 to 70%.
However, since Pd is an extremely expensive noble metal, even Agxe2x80x94Pd alloys are very expensive as compared with Ag alone. Additionally, Ag-containing noble metal alloy electrode layers such as Agxe2x80x94Pd alloys and Agxe2x80x94Pt alloys have very low heat resistance when they are thin. This necessitates to increase the content of high-melting point noble metal such as Pd or Pt to enhance heat resistance, inviting a cost increase. Further, the alloying of Ag with Pd, Pt or the like has the problem that as the content of Pd or Pt increases, the alloy increases its electric resistance and loses its performance as the electrode. In order to form a low-resistance electrode, the thickness of an alloy layer must be increased, which not only increases the amount of material used and hence, the manufacture cost of the electrode to invite a cost increase, but also exacerbates the display quality.
In addition to the problem that Ag is difficult to use alone for the aforementioned reason, another problem arises from the fact that Ag is highly reactive with lead base dielectric materials. Even when Ag is alloyed with other high-melting point noble metals, the Ag component in the electrode can react during firing of lead base dielectric material to incur a substantial increase of electrode resistance and in worst cases, line breakage. It is thus very difficult to use the alloy at a thickness as thin as 1 xcexcm or less.
Even the use of high-melting point noble metals such as Pt and Pd alone as the electrode is problematic when ceramics such as alumina are used as the substrate. Since the surface of ceramic substrates is not flat, the heat resistance of the electrode layer becomes degraded when the film thickness is less than 1 xcexcm. This allows the electrode to increase its resistance during the high temperature process involved in the formation of a dielectric layer.
The present invention addresses a thin-film EL device comprising a lower electrode layer, a lower insulating layer, a light emitting layer, and an upper electrode layer stacked in order on a substrate. An object of the present invention is to provide a thin-film EL device of high display quality at a low cost by acquiring satisfactory light emitting properties without using an expensive high-melting point noble metal in the lower electrode layer and without increasing the thickness of the lower electrode layer, even when the lower insulating layer contains a lead base dielectric material.
According to the present invention, there is provided a thin-film EL device comprising at least a lower electrode layer, a barrier layer containing a conductive inorganic compound, a lower insulating layer, a light emitting layer, and an upper electrode layer stacked in order on an electrically insulating substrate. The conductive inorganic compound is preferably an oxide, more preferably an oxide containing indium and/or tin. The lower electrode layer is preferably a metal electrode containing silver. The lower insulating layer preferably comprises a lead-containing oxide dielectric. The barrier layer preferably has a resistivity of up to 100 xcexa9xc2x7cm and often, at least 10xe2x88x924 xcexa9xc2x7cm. The lower electrode layer preferably has a resistivity of up to 2xc3x9710xe2x88x925 xcexa9xc2x7cm. The barrier layer often has a thickness of 0.02 to 0.5 xcexcm, especially 0.02 to 0.2 xcexcm.
Also contemplated herein is a composite substrate comprising an electrode layer containing silver and a barrier layer containing a conductive inorganic compound stacked in order on an electrically insulating substrate.